PRINCIPAL DETERMINANTS OF AQUATIC MACROPHYTE RICHNESS IN NORTHERN EUROPEAN LAKES

Determinants of macrophyte species richness were evaluated using data from 641 lakes in Norway, Sweden, Denmark and Finland. The species pool of aquatic macrophytes was carefully delineated prior to establishing the recorded number of species (S) for any lake. The expected richness E[S\X] for a vector of environmental variables, X, related to lake area, altitude, trophic state and several water quality variables. Neither the size of the regional species pool nor the latitude affected S appreciably. Area (A) contributed most to explain variation in species richness. E[S\A] could be modelled by a Weibull-type curve. The power function employed by the theory of island biogeography is a limiting case of the Weibull model. Residual analysis identified factors which either increased or decreased species richness. A number of cases of low species richness and low floral diversity related to lake regulation, low pH or hypertrophication. Elevated species richness occurred preferably within quite small, often mesotrophic lakes which had undergone lowering of their surface levels. Lakes exhibiting a highly assorted macrophyte flora often experience intermediate disturbance and stress from moderate alterations of their water levels (1-3 m year-1). Overall, meso-eutrophic lakes supported higher species richness than did either oligotrophic or highly eutrophic lakes.